package 笔试;

import java.util.Arrays;
import java.util.Scanner;

/**
 * @author:谢君臣
 * @Date:2021/8/1-10:38
 * @version:1.0
 * @function:
 */
public class 路灯 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int l = in.nextInt();
        int[] arr = new int[n];
        int i =0;
        double max =0 ;
        while (i<n)
            arr[i++] = in.nextInt();
        Arrays.sort(arr);
        for (int i1 = 1; i1 < arr.length; i1++) {
            max = Math.max(max,arr[i1]-arr[i1-1]);
        }
        if (arr[0]!=0) max = Math.max(max,(arr[0]-0)*2);
        if (arr[n-1]!=0) max = Math.max(max,(l-arr[n-1])*2);
        System.out.println(String.format("%.2f", max/2));
    }
}
